Conventionally, a thermal-type air flow measuring device for measuring an air flow rate by use of heat transfer between the device and air is widely known. The air flow measuring device is disposed in an intake passage leading to an internal combustion engine to be used for measuring the flow rate of intake air suctioned into the engine (flow rate of intake air may be hereinafter referred to as an intake air amount).
More specifically, this air flow measuring device takes in a part of intake air flowing through the intake passage and generates an electrical signal in accordance with the intake air amount. The air flow measuring device includes a housing that defines a bypass flow passage through which the taken-in intake air flows, and a sensor that is accommodated in the bypass flow passage to produce the electrical signal as a result of heat transfer between the sensor and the taken-in intake air. The air flow measuring device reduces the influence of turbulence of a flow of intake air in the intake passage by arranging the sensor in the bypass flow passage instead of disposing the sensor directly in the intake passage, through which intake air passes, so as to output a measurement value with few variations. See, for example, JP-T-2002-506528 (corresponding to U.S. Pat. No. 6,332,356B1), and JP-A-2003-083788 (corresponding to US2003/0046996A1).
Pulsation may be produced in the flow of intake air in the intake passage due to, for example, opening and closing of a valve of the engine. The intake air amount when the pulsation is generated in intake air changes over time, fluctuating between a larger-side peak value of the pulsation and a smaller-side peak value of the pulsation. As a result, due to the thermal-type measuring method whereby the measured value is outputted using the heat transfer relative to air, the electrical signal and the measured value are lower than a central value of the pulsation as a true value, and a negative-side error is thereby produced.
Consequently, in the air flow measuring device, by making longer a passage length L2 of the bypass flow passage than a passage length L1 when air flows straight through the intake passage without flowing through the bypass flow passage, increase ranges of the electrical signal and measurement value in accordance with a value of L2/L1 are set, and the negative-side error of the measurement value is thereby resolved. See, for example, JP-T-2002-506528 and JP-A-2003-083788.
The pulsation of intake air tends to have a large amplitude along with, for example, the recent popularization of exhaust gas recirculation (EGR), and accordingly, a backward flow may be periodically generated. In this case, because the backward flow flows into the bypass flow passage through its outlet, those which generate a negative-side electrical signal as well as a positive-side signal are employed for the sensor. Thus, the sensor can also detect the backward flow together with a forward flow.
However, even though the sensor can detect the backward flow, the negative-side error also increases due to the increase of amplitude of the intake air pulsation. Accordingly, the increase range needs to be set to an even larger value, and L2 needs to be still longer than L1. Nevertheless, since the extension of L2 involves deterioration of a pressure drop in the bypass flow passage, there is a limit on the extension of L2. As a result, measures need to be considered from the other point of view to resolve the negative-side error.